In many fields of technology substrates are put through a variety of physical and chemical treatments. The temperature of a substrate during processing may thereby be of paramount importance, while at the same time it is difficult or practically impossible to take measurements thereof using conventional methods.
A good example of this issue can be found in the field of semiconductor processing, where a wafer may be heated within a “floating wafer reactor” as described in U.S. Pat. No. 6,183,565 and marketed under the trademark Levitor® by ASM International N.V. of Bilthoven, The Netherlands. In a floating wafer reactor substrates are brought one by one and successively between two essentially flat furnace bodies parallel to the substrate, after which the furnace bodies are moved towards one another and positioned a short distance, 2 mm or less, away from the wafer. The wafer is supported without mechanical contact by gas streams emanating from a large number of passages in the furnace bodies close to its upper and lower surfaces. A floating wafer reactor may be used to subject a wafer to a so-called spike annealing treatment. An inserted wafer is thereby rapidly heated to a certain temperature at which it is kept for a relatively short time, the anneal time, which is on the order of seconds. Physically, the rapid heating of the wafer is primarily accounted for by heat conduction from the hot furnace bodies to the (relatively) cold wafer, through the narrow gaps of gas therebetween. Though heating of the wafer takes place quickly, the residence time of the wafer in the reactor may be shorter than the time needed to reach its final temperature. In order to gain better control over the process and to improve the repeatability thereof, it is desirable to be able to monitor the actual temperature of the wafer during processing. Conventional ways of temperature measurement are problematic however. Pyrometric temperature measurement, for example, requires optical access to the wafer. Such optical access is not available in the reactors that are currently being manufactured by applicant, while creating it would disturb the thermal design of the apparatus. Furthermore, pyrometric temperature measurement depends strongly on the reflectivity of the substrate and requires complex algorithms to compensate for any variations in reflectivity due to, for example, temperature dependency. Conventional temperature measurement using thermocouples on the other hand, which requires mechanical contact of the thermocouples with the wafer during processing, is at variance with the basic principle behind the system. In a Levitor reactor, wafer support is fully taken care of by the gas streams that not only heat the wafer uniformly, but also support and position it. The flow pattern of the gas streams is thus vital to the operation of the Levitor reactor, while mechanical components for temperature measurement in the gas bearing are likely to disrupt and alter said pattern.
It is therefore an object of the present invention to provide an accurate method for determining a substrate temperature during processing, whereby optical and/or mechanical contact with the substrate is not required.